If you're driving it now , at the worst it'll take one week of daily driving to get it whistle clean .
Me , I'd not trust it on a long road trip if it's running too warm now as heat rise is slow but steady , you're begging for trouble in October as it is .
I did mine in Summer and yes , I left the heater on (both knobs , 240D's have split heater core diversion) and drove with the windows open , cooking like a sausage because if you don't do it right , no point in doing anything .
Best of luck here , keep us posted , I love my 240D more than my fancier and faster Turbo OM616's .
Slow , though
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